1. Field of the Invention
The present invention relates to an optical scanning apparatus and image forming apparatus and, more particularly, to an optical scanning apparatus which uses an OFS (Over Field Scanner) optical system designed to receive a light beam emitted from a light source means within a range wider than the width of a deflecting surface in the main scanning direction and optically scan a photosensitive member and is suitably used for an image forming apparatus such as a digital copying machine or laser beam printer, and methods of manufacturing the optical scanning apparatus and image forming apparatus.
2. Related Background Art
Recently, as digital copying machines, LBPs (Laser Beam Printers), and the like become faster in operation, an OFS optical system which can perform scanning with an increased number of deflecting surfaces (reflecting surfaces) of a polygon mirror serving as an optical deflector, a multi-beam scanning optical apparatus based on a scheme of simultaneously forming a plurality of scanning lines with an increased number of light sources (emission points), and the like have been proposed. In addition, a faster apparatus can be implemented by combining the above two types of techniques.
In an OFS optical system, part of an incident light beam having a Gaussian intensity distribution is cut by a deflecting surface of a polygon mirror to form a scanning light beam (reflected light beam). For this reason, the scanning light beam has an intensity distribution, which in turn forms a scanning line illuminance distribution on a scanned surface, resulting in the density irregularity of an image.
Conventionally, therefore, as disclosed in Japanese Laid-Open Patent Application No. 11-14923, an ND filter with gradation serving as a light amount adjusting means for adjusting the exposure on a scanned surface is disposed between a light source and an optical deflector and is designed to be movable in the gradation direction and rotatable in a plane including the filter surface, thereby reducing the intensity distribution of a scanning beam after deflection.
Japanese Laid-Open Patent Application No. 9-96769 discloses a technique of correcting the asymmetry of a light amount distribution by shifting or tilting a light source in the main scanning direction with respect to a collimator lens.
In this method, the light source is shifted from the optical axis of the collimator lens to cause a parallel light beam emerging from the collimator lens to emerge obliquely with respect to the optical axis of the collimator lens. With this operation, the light beam incident on a polygon surface is shifted in the main scanning direction, thereby making adjustment such that the center of the intensity distribution of the light beam coincides with the center of the polygon surface.
In this method, however, since the light source is moved relative to the collimator lens, a field angle is set, and a focus error occurs at the same time. The influences of these phenomena are enhanced as the focal length of the collimator lens is decreased to increase the use efficiency of light from the light source.
In general, the focus adjustment sensitivity in the main scanning direction is proportional to the square of the lateral magnification of the overall system, and the focus adjustment precision of the light source and collimator lens is about 5 μm. To prevent a focus error in the light source, a high mechanical precision is required, resulting in an increase in cost.
In addition, since the collimator lens and light sources are discrete components, if the light source fails, the optical scanning apparatus must be replaced, requiring a high cost in terms of market serviceability. If the light source and collimator lens are integrated into one unit, a deterioration in this light source can be coped with by replacing the unit with another.
Although optimal filters for the design center values of an optical system can be prepared, the influences of assembly tolerance, variations in parts, and the like are too large to be neglected. If an assembled semiconductor laser chip is tilted in the main scanning direction with respect to the optical axis, in particular, the intensity center of a substantially parallel light beam emerging from the collimator lens shifts from the optical axis center. As a consequence, the intensity distribution of an incident light beam cut by the polygon mirror becomes asymmetrical. In an extreme case, the intensity peak of a scanning line formed on a scanned surface is located outside the effective scanning range, and reaches a value twice or more the initial value upon tilting of the intensity distribution. In such a case, no stable effect can be expected even with measures such as a filter.